Can myofacial dental pain/throbbing post root canal be due to old spinal injury? Q. Having more dental work done now. Will happen with fillings?

No. The nerves to your teeth come from the trigeminal ganglion inside the skull, not from spinal nerves. That stated, some people with chronic pain on chronic opioids are more sensitive to pain coming from an acute new event than someone not on opioids. This is called opioid induced hyperalgesia.
Too convoluted. Basically no. But. Depending on if you have a nerve damage sensitisation going on in the head and neck areas, it can make you more at risk for other neurogenic pain from procedures in that region. That would not be considered "myofascial" (soft-tissue), but more nerve involved.
Negative. Highly unlikely. These are almost assuredly unrelated events.
No. Myofacial pain in the area around the jaw it not likely related to injury of the spine.
No. But. Dental pain post root canal not due to old spinal injury. Your tooth after root canal should not be throbbing badly. Contact dentist who did root canal. Alternately, myofascial pain of the head, neck and shoulder CAN be related to old upper spinal injury. See orofacial-TMJ expert about head and neck myofascial pain.